[API] Subscribe with arguments (real time filtering)

There should be a way to pass arguments to subscriptions so that only a subset is subscribed.

Model schema -

```graphql
type MyData @model {
id: ID!
x: Int
other_data: String
}
```

Subscription method `onUpdateMyData` which take an input parameter `x` -

```graphql
type Subscription {
onUpdateMyData(x: Int): MyData
@aws_subscribe(mutations: ["updateMyData"])
}
```

Mutation method `updateMyData` -

```graphql
type Mutation {
updateMyData(input: UpdateMyDataInput!, condition: ModelMyDataConditionInput): MyData
}
```

Following the [official documentation](https://docs.amplify.aws/lib/graphqlapi/subscribe-data/q/platform/android), I wrote the below code for subscription. Works as expected, i.e, gets triggered whenever the `updateMyData` mutation is called, irrespective of the value of x.

```kotlin
Amplify.API.subscribe(ModelSubscription.onUpdate(MyData::class.java),
{ Log.i("ApiQuickStart", "Subscription established") },
fun(onUpdate: GraphQLResponse) {
if (onUpdate.hasData()) {
Log.i("ApiQuickStart", "Update: " + onUpdate.data.toString())
} else if (onUpdate.hasErrors()) {
Log.e("ApiQuickStart", "Error: " + onUpdate.errors.joinToString { it.message })
} else {
Log.i("ApiQuickStart", "Null update")
}
},
{ onFailure -> Log.e("ApiQuickStart", "Subscription failed", onFailure) },
{ Log.i("ApiQuickStart", "Subscription completed") }
)
```

But the problem is, I only want to subscribe for a specific value of x, say 5.
So, I tried by modifying the request -

```kotlin
ModelSubscription.onUpdate(MyData::class.java).apply {
putVariable("x", 5)
}
```

This made no difference. I'm still receiving updates for other values of x.

This is how I call `updateMyData` mutation from AppSync console -

```graphql
mutation {
updateMyData( input: {
id: 1,
x: 5,
other_data: "some updated info"
} ) {
id, x, other_data
}
}
```

Subscription in AppSync console works like a charm, i.e, it gets triggered only for a subset where `x = 5` -

```graphql
subscription {
onUpdateMyData(x: 5) {
id, x, other_data
}
}
```

Just to mention, DynamoDB is used as the data source.

Start at ModelSubscription.onUpdate and Amplify.API.subscribe, then compare the generated request with the AppSync subscription that uses onUpdateMyData(x: 5). Trace whether putVariable("x", 5) reaches the subscription request and confirm that updates with other x values are excluded; verify against the DynamoDB-backed example described in the issue.

Written by the indexing model from the issue text.